The invention described herein relates to a leaf spring type of collet especially designed for pulling fuel rods into grids spaced along the length of a nuclear reactor fuel assembly.
A typical fuel assembly for a nuclear reactor includes a square array of fuel rods held in spaced relationship with each other by a series of grids of egg crate configuration placed along the fuel assembly length. Each cell in the egg crate grid holds a fuel rod and the four walls of each cell contain springs and projections which hold the rod in a set axial position. Since the springs and projections present substantial resistance to the insertion of fuel rods into the grids, special designs of fuel rod pullers are used to pull the rods into the grid cells at the time of assembly.
In prior designs, this has been accomplished by manually connecting the end of a pull rod to an externally grooved end plug of particular design welded in the end of a fuel rod. The connector between the pull rod and end plug is formed with two semi-cylindrical mating shells having an internally projecting flange located on each end which fit into complementary grooves respectively formed on an end of the fuel rod end plug and on an end of the fuel rod puller. The shell diameter is the same as the outer diameter of the fuel rod thus facilitating the drawing of a feul rod into its cell in the fuel assembly grid.
The major disadvantage of this prior art design is that it is inefficient. The operator positions both halves of the shell connector on adjacent ends of the fuel rod and rod puller and then pulls the fuel rod in a direction to create a friction fit and thus hold the shell halves on the fuel rod and rod puller. Although this manual action is in itself simple and capable of being quickly done, it nevertheless still involves an expensive labor operation because of the large number of fuel rods to be pulled into an assembly. A 17 by 17 fuel assembly will contain slightly over 200 fuel rods thus requiring more than 200 separate operations in preparing the fuel rods for loading into the assembly grids. After each fuel rod is pulled into its cell in the grid, the shell halves must be removed thus almost doubling the amount of labor time necessary to effect assembling the fuel assembly. It therefore is apparent that the need exists for an improved design of fuel rod puller which will act automatically not only to pull a fuel rod into a fuel assembly but also return it to its starting point without engaging in any substantial kind of manual effort in carrying out this part of the loading process.